Mike Brown's Planets

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A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

In the months since I first posted about the potential hotspot on Europa associated with a potential plume on Europa, I’ve been refining
our computer model and digging deeper into trying to understand what is going
on. As you’ll remember from the last post, a potential plume spotted on Europa
looked like it might be coming from a spot that the Galileo spacecraft had
earlier shown was hotter at night than it should be. We discussed two potential
explanations for this night time hot spot. The more exciting explanation was
that the spot in question could be experiencing excess subsurface heat flow due
to recent or ongoing geologic activity, as one might expect from an area with
potential active plumes or geysers or volcanoes or whatever. The other
possibility was that the spot may be hot at night due to its specific thermal
properties, particularly its thermal inertia. A high thermal inertia could keep
the location warm during the night, but it would also make the same spot harder
to heat up during the day – think about how pavement stays warm after a hot day
long after the sun has done down but is also cooler than it should be in the
morning. A spot actively heated by geologic activity, in contrast, would
maintain elevated temperatures throughout the day-night cycle.

With only the Galileo night time temperature measurements,
there was no way to know which of these two scenarios was occurring. Luckily,
we have recently obtained daytime temperature measurements using the new
massive new ALMA telescope in Chile. Our daytime ALMA observations allow us to
tell the difference between these two scenarios. We left you last time with the
puzzling observation that the potential hot spot was actually a little colder in the ALMA daytime image than
our model predicted. After extensive testing and refinement of the model, that
finding remains true. Here is our updated data-model comparison.

The location of the proposed hot spot is indicated by the
white circle and, relative to our model, it is cold during the day and hot at
night. At first glance, this pattern seems more like a potential thermal
inertia anomaly than an active hot spot. To look a bit closer, we modeled the location
throughout the Europa day to better examine the day-night temperature profile
and see what it would take to fit both the Galileo and ALMA temperatures. Below you can see our three modeled scenarios.

The green curve is our model’s predictions for the proposed
hot spot. Like you saw in the first figure, it underestimates the nighttime
temperature from Galileo on the left and overestimates the daytime temperature
from ALMA on the right. To test the hypothesis of subsurface heating, we
increased the heat flow in our model, which produced the red curve. In this
case, the amount of extra heating needed to match the Galileo nighttime
temperature created a daytime temperature that is much higher than we observe
with ALMA. However, when we simply increased the model thermal inertia (with a
small albedo adjustment within our uncertainties), we were able to fit both
temperatures well. Sadly, this suggests that the potential hot spot associated
with the potential plumes is most likely just a spot with a higher than average
thermal inertia, making it especially good at retaining daytime heat into the
night.

You might rightly be wondering why this one spot should have
such a relatively high thermal inertia. The answer could be because of its
proximity to Pwyll, the biggest, freshest crater on Europa. Pwyll Crater is
just below and to the right of the proposed plume location and, interestingly,
is even more anomalous. It is also cold during the day, and it is the big,
obvious red anomaly on the night side. So, it is not just the proposed plume
source that appears to have an elevated thermal inertia, but the entire Pwyll
Crater region. This could be because material ejected during crater formation
is blockier than the rest of the surface, so that it acts more like rock than
sand. It’s also possible that the impact exposed purer water ice, allowing
sunlight to penetrate deeper into the surface in this area. That sunlight would
be stored as heat below the surface, which would be released slowly at night,
mimicking the effects of a high thermal inertia. Really, we don’t know for sure
what would cause the elevated thermal inertia, but it looks like the
possibility of subsurface heating is unlikely.

So the purported hot spot is still unique, but not so hot.
What does this mean for the plumes? Our observations do not specifically
address the existence or nonexistence of the plumes. They do, however, suggest
that the proposed detections are not associated with an active hot spot, which
would have otherwise made the potential plume detections much more convincing. In
the end, we still don’t know, but we are excited about what else the ALMA
datasets might tell us about the surface.

Europa is hot right now. With the planning for the Europa Clipper mission underway and talk even of a lander, scientists are paying more attention to the little icy satellite than ever. Much of the recent excitement has been a discussion of the now-you-see-them-now-you-don't plumes that might be jetting material from the interior ocean. Such a possibility would be quite exciting indeed, as it would allow us to understand the interior conditions of Europa without having to do something crazy like dig a hole through the ice. But the plumes have been on the tantalizing edge of detection so far.

Today another tantalizing bit of evidence came out. One of the possible plumes was possibly seen again in the same place. The new detection, like all of the old ones, is still just on the verge of being believable. I personally remain skeptical, but I would call it skeptical but hopeful rather than skeptical and dismissive.

While I have no extra insight into the plumes, one of the really interesting things about the potentially-repeating plume on Europa is where it is seen. As mentioned today, old data from the Galileo spacecraft suggests that the possible source location of the possible plume is possibly a spot on Europa that is hotter than it should be. Hotter than it should be! This is very exciting, because it would both suggest that liquid water is probably close to the surface, but it is also something that I can actually address.

For the past year my graduate student Samantha Trumbo and I, along with our colleague Bryan Butler and NRAO have been mapping the surface temperature of Europa from the new ALMA millimeter telescope in Chile. So is there really a hot spot on Europa????????

The answer is complicated. To know if a spot on Europa is hotter than it should be we have to figure out how hot it should be. This requires a lot of work and is part of Samantha's Ph.D. thesis. We don't have all of the answers yet, but inspired by the news today she took a careful look at that spot on Europa. This is all still very preliminary work. We would normally wait to show results until our analyses were complete. But with the excitement of a potentially plume and hot spot we though we would give a quick example of what our data from ALMA show.

I'll let Samantha tell the rest of the story:

The recent Hubble Space Telescope
(HST) observations of Europa show a recurring anomaly that may be consistent
with a water vapor plume. The scientists have located the most likely surface
source region for this feature and found that it coincides with a potential
hotspot seen in temperature data from the Galileo orbiter. If this hotspot is
real, this could be strong evidence that the HST anomaly is really due to
geologic activity on Europa, rather than some less interesting cause.

We've observed Europa using the Atacama Large Millimeter/submillimeter Array (ALMA)
and obtained complete temperature maps of the surface. These can also be used to
look for hotspots that may be indicative of geologic activity or even plumes. However,
the thermal maps are strongly influenced by the surface properties of Europa,
particularly in how well the different surface materials absorb light, radiate
heat, and resist changes in temperature. Therefore, in order to look for
hotspots, we developed a computer model to try to account for these variables.
The model calculates the absorption and re-radiation of sunlight, as well as
the day-to-night cycle of heat flow into and out of Europa’s near-surface
layers. This allows us to create model ALMA images that we can compare to our
observations in search of anomalies.

One of our observations captures the
inferred source region of the potential plume signature captured by HST. When
we compare this observation to the results of our thermal model, the region
does not stand out as anomalously warm. In fact, it is a little colder than our model predicts. Critically, though, we look at Europa during Europa's daytime (because we can't do otherwise). The Galileo spacecraft, however, could sit behind Europa and look at its night side, where the surface should have cooled. While most of the surface has cooled, that one spot hasn't cooled very much. The area near the purported plume source,
which also includes the large Pwyll impact crater, stands out as much warmer
on the night side than the model predicts.

The top row shows temperatures on Europa measured from ALMA compared to
our expectations ("thermal model"). The circle at about the 4 o'clock
location is the alleged hot spot. The far left shows the difference
between the measurements and our expectations. As you can see, our
computer model is not yet perfect, but it does a pretty good of
predicting the temperature of Europa. The bottom row shows the same
computer model but now comparing to the Galileo nightside data (note the
change in temperature scale; its a lot colder at night). Again, the
match is not too bad except that in one place the nightside temperature
is ~15 degrees C hotter than it should be. That is a huge difference. In
fact, the dayside and nightside temperature at that one spot are nearly
the same. Why? Maybe there is hot material just beneath the surface.
Maybe the material retains heat better, like hot pavement on a cool
night. Further analysis should answer the question.

Something that is warm at night is definitely a hot spot, but why is it hot? It’s possible that spatial
variations in the surface properties could cause localized areas to appear anomalous
at night. Indeed, the Galileo team noted that the region in the vicinity of
Europa’s large Pwyll crater was hotter than expected and suggested that a large
thermal inertia may be to blame, although they did not rule out the possibility
of subsurface heating. Thermal inertia is essentially the ability to resist
temperature change. Sand, for instance, has a low thermal inertia; it gets very
hot during the day, but quickly becomes cold at night. Rocks, on the other hand,
have a much higher thermal inertia; they can remain warm well into the evening.

Just knowing that the spot is hot at night can't tell you why. But the good news is that the combination of daytime temperature and nighttime temperature will allow us to answer the question. We plan on refining our model to see if it is possible to explain this hotspot
in the Galileo data with variations in thermal properties of the surface
materials, but at this time we cannot rule out the intriguing possibility of subsurface
or plume activity. We will also use our data to search for other potential
hotspots, which may not have been visible during the HST observations. Stay tuned for more excitement.

I know, I know.We
have all been instructed by Arthur C. Clarke to attempt no landings on Europa.
But if you did land on Europa,
wouldn’t you like to know where to go? If you do, my graduate student, Patrick
Fischer, has a paper coming out that you probably want to read.

First, perhaps, it might be best to understand why anyone
would want to land on Europa at all. Europa – the second of Jupiter’s four
large satellites – is clearly a special place. Ever since the time of the
Galileo spacecraft nearly 2 decades ago, we have recognized that Europa’s fresh
icy surface, covered with cracks and ridges and transform faults, is the
external signature of a vast internal salty ocean. If, on a whim, you climbed
down a crack on the surface of Europa and made your way down into the ocean
(which, interestingly, might be something you actually could do; though it is
more likely you would get stuck and squeezed to death; hard to tell) and then
you figured out how to swim down to the rocky bottom something like 100 km
below the base of the ice (a depth 10 times greater than the Marianas Trench,
by the way) you would instantly be able to answer what to me is one of the most
interesting mysteries about Europa. What is happening at the boundary of the
rocky core and the ocean? The answer has profound effects on the type of world
that Europa ultimately is.

What might be happening down there? The least interesting
possibility is that the bottom of the ocean is a stagnant, inactive place:
water on top; rock on bottom; a little dissolution of the rock into the water
in between, but, otherwise, with not much going on. A world like this wouldn’t have much of a
source of chemical energy in the ocean, and it’s hard to imagine it could
support even the most elementary types of life. If you had taken all of that
effort to swim all the way to this cold dark dead ocean bottom, you might start
to ask yourself whether or not it was even worth it. The most interesting
possibility – at least the most interesting possibility that I can think of –
is that the rocky bottom of the ocean is almost like a miniature Earth, with
plate tectonics, continents, deep trenches, and active spreading centers. Think
about mid-ocean ridges on Earth, with their black smokers belching scalding
nutrient-rich waters into a sea floor teaming with life that is surviving on
these chemicals.It doesn’t take much of
an imagination to picture the same sort of rich chemical soup in Europa’s ocean
leading to the evolution of some sort of life, living off of the internal
energy generated inside of Europa’s core. If you’re looking for Europa’s whales
– which many of my friends and I often joke that we are – this is the world you
want to look for them on.

Sadly, this is not Europa

Sadly, no one is going to climb down through a crack and
then swim to the bottom of Europa’s ocean for a long long time, so this is
where landing on the surface comes in. If the chemicals that are dissolved
inside of the ocean could somehow make it to the surface, we could learn a lot
about what is going on deep inside of Europa just by analyzing a little a
sample of the surface.

OK,then, let’s go land! But where? You probably only get one
shot at a lander, and you probably don’t get to move once you land, so you had
better pick the right spot. The announcement a couple of years ago, that plumes
of water jetting from Europa’s south pole had been discovered by the Hubble
Space Telescope, seemed to have answered the question: land at the pole, and
wait for plumes to rain down upon you (or, perhaps even more easily, fly
through the plumes and collect samples without even landing!). The bad news,
however, is that the plumes now appear to be elusive at best and non-existent
at worst. Since their initial detection no one has been able to see them again.
Are they (very) sporadic? Was the initial detection an unfortunate spurious
signal that was misinterpreted? No one yet knows, but no one today is going to
count on plumes for measuring the chemical composition of the ocean.

Luckily, our new paper shows that we don’t need plumes to
sample the interior, and we even conveniently point out a potential landing
area that is large enough to easily target with your favorite lander.

First, how do you find a landing site? What we are actually
doing is simply mapping the composition of the ices across the surface of
Europa. Such mapping has been going since the time of the Galileo mission, but
with modern telescopic instruments and high spatial resolution adaptive optics
systems on large telescope on Earth, we can do a better job of making global
scale maps than the Galileo spacecraft ever could. In the earlier Galileo
mapping efforts and in our own early analyses of our own data, we concentrated
mainly on dividing the surface of Europa into an ice component and a non-ice
component and then trying to figure out what the non-ice component was. Like
the earlier Galileo analyses, we found that the dominant non-ice component is
sulfuric acid that is created when sulfur (ultimately derived from volcanoes on
Io!) bombards the water ice on the surface of Europa. We also found, though,
that some of the non-ice material was magnesium sulfate – Epsom salts, in fact
– which we suggested indicated a magnesium source coming from inside of
Europa’s ocean that then mixes with the incoming sulfur.

Patrick Fischer, in his new analysis, decided to take these
ideas one step further. He wanted to know if there is anything else on the surface
of Europa besides just the water ice and the sulfur products. To do so, he took
the spectra of nearly 1600 separate spots on the surface of Europa and started
looking for anything unusual that stood out. The answer was…… maybe. Staring at
that many spectra you’re bound to find something to catch your eye. He needed a
more rigorous method to group the spectra together, and eventually he developed
a very clever new mathematical tool which allows you to take an arbitrary
collection of spectra and automatically, with no preconceived human biases,
classify them into an arbitrary number of distinct spectra, and present maps of
where those materials are present on the surface. When he asked the tool to
give him to find the two most distinct spectra on the surface of Europa, he
reproduced the ice plus sulfur products distributions that had been known for
decades. When he asked for a third distinct spectrum, though, a large region on
the surface of Europa suddenly popped out as being composed of material unlike
the ice or sulfur products of the previous map.Staring back and forth between the composition map he had just made and
a geological map of the surface of Europa, he was startled to realize that he
had nearly precisely mapped out one of the largest regions of what is called
“chaos terrain” on Europa.

Mapping the composition of the surface of Europa has shown that a few
large areas have large concentrations of what are thought to be salts.
These salts are systematically located in the recently resurfaced "chaos
regions," which are outlined in black. One such region, named Western
Powys Regio, has the highest concentration of these materials presumably
derived from the internal ocean, and would make an ideal landing
location for a Europa surface probe

On Europa, "chaos terrains" are regions where the icy surface appears to
have been broken apart , moved around, and frozen back together.
Observations by Caltech graduate student Patrick Fischer and colleagues
show that these regions have a composition distinct from the rest of the
surface which seems to reflect the composition of the vast ocean under
the crust of Europa.

Chaos terrain was noticed early on in the Galileo mission as
regions which look like the surface of Europa has become cracked and jumbled
and – intriguingly – perhaps even melted in recent times. If you had to vote
for a location on Europa where ocean water had recently melted through and
dumped its chemicals on the surface, you would vote for chaos terrain. And now
Patrick had found that on large regional scales chaos terrain has a different
composition than the rest of the surface of Europa!

And what do the spectra tell us that the unique composition
of this chaos terrain is? Sadly, we can’t yet tell. To date, we have not found
unique compositional indicators in the spectra of this region, though our
search is ongoing. Our best bet, though is that we are looking at salts left
over after a large amount of ocean water flowed out on to the surface and then
evaporated away. The best analogy would be to large salt flats in desert
regions of the world. Just like these salt flats, the chemical composition of
the salt reflects whatever materials were dissolved in the water before it
evaporated. On the Earth, salt flats can contain any number of exotic salts,
depending on the surrounding rock chemistry. On Europa, the salts will tell
about the rock chemistry, too, though the rock is the material far below at the
base of the ocean.

We think, then, that we have found a giant salty
patch on the surface of Europa, and very likely the region of most recent
resurfacing and undisturbed chemistry. I have tried very hard to get Patrick to
call this salty patch Margaritaville, but he does not think that graduate
students are quite established enough to make jokes like that. I’ll make it for
him, though. And I will tell you: attempt a landing there! Margaritaville will
not only have salts that tell you about the rock-ocean interaction, but it will
also have samples of everything else that the ocean has to offer. Is there
organic chemistry taking place in the oceans? Look in Margaritaville. Carbonates?
Margaritaville. Microbes? Definitely Margaritaville. All of these are best searched
for with the types of instruments currently roving around on Mars, where you
grab a sample, put it into a machine, and read back out the chemical composition.
But don’t forget to bring the cameras along, too, just to see what else is
lying around.The jumbled and exotic icy
terrain is bound to be a spectacular site up close. You might get lucky and see
a plume shooting off into the sky in the distance. And maybe, just maybe you’ll
even find a few whale bones lying around.

Ten years ago today I came in to the same office I’m in at
this moment, sat down in the same chair I am sitting in now, probably stared
out the window at the clear blue sky much like I’m doing right now. It’s even
likely that I drank coffee out of the very cup I’m drinking out of. Other than
that, though, nothing was the same. Just a week earlier, on Dec 28th
2004, I had discovered the second brightest object that we had ever seen in the
Kuiper belt (the brightest, of course, being Pluto). We didn’t yet know how big
it was so my mind kept spinning with possibilities. Maybe it had a dark comet
like surface and so to be so bright it had to be really big! Maybe as big as
Pluto! Maybe bigger! (The object, now called Haumea, is now known to be about a
third of the mass of Pluto and one of the strangest objects in the outer solar
system).

Perhaps even more exciting, I had discovered the object
while re-processing old images that I had taken a few years back. There was
another year’s worth of images to re-process. Maybe there would be more!

I first published this five years ago today. It's all still true. -- MEB

My father was a rocket scientist. Well, OK, not precisely. More
specifically he was a rocket engineer. Or, more precisely still, he was
an engineer who worked on the computers that went into space and
navigated the rockets. He worked on the Saturn V that lifted Apollo
astronauts toward the moon, he worked on the Lunar Module, which touched
down on the moon, he worked on the Lunar Rover, which drove astronauts
around on the moon. All of this before he was 30 years old.

I never remember him talking about it at all, talking about what it was
like to send men to the moon, to be involved in such a tremendous
adventure, but, ten years ago, in the little farming town on the edge of
the Mississippi River where he grew up, I had a conversation with one
of his friends from those days, and he told me that they all felt like
they had lived in a magical time. After the Apollo missions ended, they
all later worked on the Space Station and more mundane things like the
ticket-taker on the BART trains that I used to take when I was a
graduate student living on the San Francisco Bay. But nothing in their
lives was ever quite like a being a bunch of thirty-year-old kids living
in northern Alabama having the blind optimism to think that if there
was a rocket being built they knew enough to put the computers together
to make those rockets bring people to the moon. And back. And then
actually doing it.

It didn’t snow much in northern Alabama where I grew up, so,
when I went to college further north, I was at a serious disadvantage when the
first blizzard came through and everyone streamed out of the dorms to engage in
an all night snowball fight. After my first rounds of fusillades ended up
splintering to little wispy bits in midair I quickly got the hang of
compaction, looking for wetter snow, and doing what I could to increase the
density of the snowballs. I broke a window, confessed, and escaped punishment
with the lame but true excuse that I had no idea snowballs could break windows.
Friends with more snowball experience and more delinquent childhoods told me
about burying a stone or two inside of the snowball to increase its destructive
power.

These look too fluffy to me. I don't think they'd survive flight.

I don’t get much snow in southern California, but I do spend
a lot of my time thinking about those early snowball experiences and about the
snowball fights that have made the objects of the outer solar system.

When we moved into our house more than 7 years ago now the old owners left their Dish Network satellite TV dish attached to the roof. A few months later we got a sternly worded letter from the Disk Network demanding that we send them the dish back. With my detailed knowledge of the intricacies of the American legal system my obvious response was: come and get it. Which would have been fine with me. But, actually, that was not even my response, my response was to throw the letter in the trash while thinking in my head "come and get it."

Seven year later the dish was still on the side of the house. Luckily it is on the side that I never really see, so I didn't worry about it, but every now and then I thought to myself: "I should at least go up and take down that eyesore." But I never did. Until now.

I occurred to me a while ago that a parabolic dish like that would make a fine radio telescope (OK, it will end up a microwave telescope, but we'll get into the details later).

I'm not a radio astronomer or an electrical engineer or a Ham radio guy or any of that stuff, so I really had no idea what I was talking about, but it seemed a fun project for Lilah and I to play around with for the summer and for both of us to learn a little bit about microwaves. The caveat, though, is that my electronic explanations might not be exactly right. And I might break things.

We started last week. Step 1: remove the dish from the roof and see what was there. I had to snip the coax cables that went into the house and then undo five big screws and then everything just came unceremoniously down. The main issue was figuring out how to hold the wrench, dish, and ladder at the same time without falling. Luckily I survived this crucial part. Lilah stayed far enough away to avoid getting a dish on her head but to be able to both take pictures of me and make fun of me each time I dropped something and had to go pick it up.

Quick: name the three largest known objects in the Kuiper
belt. If you’ve been paying close attention you will instantly get Eris and
Pluto, and, if pressed, you will admit that no one knows which one is bigger. And
the third? An unscientific poll of people who should know the answer (my
daughter, my wife, my nephew) reveals that not a single one does.

The answer, of course, is Makemake (you remember how to pronounce this, right? Mah-kay-mah-kay, Polynesian style). Makemake was discovered just months after the
discoveries of Eris and of Haumea, and all were announced within days of each
other. Eris and Haumea had important stories immediately attached to them (Eris
was as big as Pluto! Haumea had suspicious discovery circumstances!), so poor
Makemake stayed in the shadow of its more famous contemporaries. It was so
overlooked that, in the hastily called press conference in which we announced
the discoveries, I couldn’t even remember the official designation of Makemake
when asked (it was 2005 FY9, of course; how could I have forgotten that?).

Sometimes I like to write about things in the sky that I've been studying. Sometimes I like to write about scientific discoveries in the outer solar system. Sometimes I even write about wild speculations I have about the solar system. But, every once in a while, I get to just sit back and watch the sky go by.

I love comets. When I first started graduate school to get my Ph.D. in astronomy, I wanted to study the most distant galaxies in the world. But my Ph.D. advisor really wanted me to start by doing a project studying a comet (actually, he wanted all of his graduate students to start with comets, because no one stuck with them; they jumped to galaxies as fast as they could). I fell in love with comets. Mostly, I think, I fell in love with the fact that you could use huge telescope to study things in the sky that you could actually see with your eyes or with binocular or with a camera. Things that were real.

So I was pretty excited about the prospect of Comet Panstarrs close to the tiny tiny crescent moon tonight. We have a great western horizon from my house and I was pretty sure we would have good views. Scientifically, I have nothing at stake. I'm not involved in any attempts to look at the comet with telescopes big or small, on the ground or in space. I just wanted to see it.

So I waited.

The tiny crescent moon was going to be easier to see, so up and down, back and forth, with binoculars I searched. THERE! It was, 25 minutes after sunset, higher than I thought. This was good news. It would be a good ~30 minutes before the comet set. Long enough that even my daughter Lilah would be able to see it.

(Lilah uses a placemat every day that has astronomy pictures [including, yes, Planet Pluto. It was a present. Really] on it, including comets. She is really really excited about seeing one in real life).

I had set out the camera and tripod earlier, and started taking long exposures, hoping to capture the comet. I kept seeing something. Maybe. To the left. Where I knew it should. Be. But? Well? I dunno.

Until, finally, jackpot:

See it? Barely? Something like 6 lunar diameters to the left of the moon?

The first thing that you notice when you look at a spectrum
of Europa -- from the Earth, from a spacecraft, it doesn’t really matter – is
the ice. Ice is everywhere. The spectrum of ice is a very distinctive looking
thing, with a quickly recognizable pattern of regions where the sunlight
reflects strongly from the surface and regions where there is less reflectance
(and remember the regions here means spectral regions, which means,
essentially, we stare at one small spot on the surface, put the light through a
prism to spread it all out, and see which colors of the rainbow are present and
which are absent. In our case our rainbow is in infrared light that your eye
can’t see, but the idea is still the same).